In ultrasonic diagnostic imaging system, spatial compounding technique is often used to reduce speckle noise, acoustic shadow and reverberation of the ultrasonic image and improve contrast and signal-to-noise ratio. As a result, the definition and readability of the ultrasonic images are improved. The spatial compound imaging technique typically images an object from multiple angles by steering scan beams. FIG. 1 shows a view of spatial compounding by using a component image C(i, j) of a linear array probe at three steering angles, wherein C(i, j) denotes the jth component image during the ith steering scan cycle (time taken to image an object three times at three steering angles is defined as a steering scanning cycle). It can be seen that the component images at the three steering angles overlap in a trapezoid area of the whole scan region. Each component image at a different steering angle shows different speckle noise and acoustic shadow and reverberation. Therefore the effect of these disturbances can be efficiently reduced by compounding the three component images, thereby achieve a better image quality in the overlapping area.
FIG. 2 is a block diagram of a typical ultrasonic imaging system, which realizes a B-mode spatial compound imaging system. A compound imaging controller in FIG. 2 determines the steering angle for scanning the object and the number of component images, and controls the steering scan by controlling the transmitting beamformer and receiving beamformer. The received signal is dynamically filtered to extract valid frequency components of the echoes. The signal-to-noise ratio of the echoes is improved and then the envelope information of the echoes is extracted. Envelope extraction followed by log-compression and down-sampling etc. is applied to produce component images at different steering angles. The newly acquired component images and the component images previously stored in the memory are subjected to an image compounding process to produce a compounded image, which is fed to a monitor for display after digital scan conversion (DSC). It is apparent that the system can perform log compression on the compounded image after image compounding process. However, the stored component images will require a relatively large storage bandwidth to prevent the images from distortion because the B-mode images have a large dynamic range if the images are log compressed after the image compounding process. On the contrary, the storage requirement can be efficiently reduced if the images are log compressed before compounding the images.
A simple real time spatial image compounding method and device is proposed in the U.S. Pat. No. 6,126,599 by James R. Jago et al. In accordance with this method, an ultrasonic imaging system acquires and stores a plurality of component images at different steering angles; the acquired component images are compounded to generate a first compounded image; one of the component images is subtracted from the first compounded image to generate a partially compounded image; thereafter when a new component image at a new steering angle is obtained, the partially compounded image is added to the new component image to form a second compounded image. That is, this method implements real-time spatial compounding by subtracting a previously acquired component image at a steering angle from a previously compounded image and then adding to a newly acquired component image at the same steering angle to obtain a new compounded image. The advantage of the method lies in that it does not need to access all the component images in the memory each time the compounding is performed, as a result the bandwidth required for memory access is reduced.
Although the above discussed patent can display image at a real-time frame rate, however, there exist the following problems: (1) the first compounded image Co(0,2) is generated only after all the component images Co(0,0), Co(0,1), Co(0,2) at all the steering angles have been acquired, as shown in FIG. 3; (2) it has to pre-generate and store a partially compounded image, which usually requires a large bit-width for storing; (3) there is no way to detect and reduce the motion blur inside the compounding loop.